The present invention relates to an improvement of a program shutter including shutter blades having the function of diaphragm blades which are driven to be closed by a cam member operated by an electric motor.
Heretofore, a program shutter including a single blade member having a diaphragm function and a shutter function was adopted in many small cameras. Particularly, the present invention assumes a mechanism in which the blade member is driven to be closed by a cam member operated by an electric motor such as a stepping motor. When the single blade member has the diaphragm function and the shutter function, the blade member is required to control both aperture value and an exposure time.
In order that the blade member attains the diaphragm function, generally, there is adopted a method of restricting the movement limit of a blade opening and closing member, which is coupled with the blade member and urged to open the blade member.
Further, in order that the same blade member attains the shutter function, there is adopted a method in which an exposure controlling timer is started from the time that a blade position detecting means such as, for example, a photo-interrupter, detects that the released blade member has been moved just before a position in which a pin hole is formed, and the blade member is driven to be returned to its initial position in response to the completion of the timer.
Accordingly, with the mechanism adapted to close the blade member with the cam member operated by the stepping motor, when the timer for the exposure control is up after the blade member has been opened by releasing the blade opening and closing member, the cam member driven by the stepping motor is moved on the moving line of the blade opening and closing member to restrict the movement limit of the blade opening and closing member, and the cam member is further moved to return the blade member to its initial position.
However, in the above-described hitherto known control system, there occurs a problem that a time lag, until the cam member actually restricts the movement of the blade driving member after the cam member has started its movement, varies in accordance with a desired aperture value.
More particularly, since the moving range of the blade driving member is large in order to increase the aperture when the brightness of an object to be photographed is low, the time lag until the cam member comes into contact with the blade driving member after the cam member has started its movement is relatively small, whereas since the moving range of the blade driving member is small in order to make the aperture small when the brightness of the object to be photographed is high, the time lag until the cam member comes into contact with the blade driving member after the cam member has started its movement is relatively large, and accordingly the exposure is too great.
In order to heretofore solve this problem, a method was adopted in which a detection means such as a photo-interrupter for starting the timer was disposed near the initial position, as compared with a position where the shutter blade actually forms the pin hole, so that a start timing of the timer is advanced.
In this method, however, since the timer is started in an area in which the movement of the blade member is not stable, immediately after the blade member has started its movement, there is a problem that a time difference until the blade member actually forms the pin hole after the timer has been started tends to be increased, and hence exposure error is apt to occur. Particularly, there is a problem that the exposure error is apt to be large when the brightness is increased, since the absolute value of the exposure time is short.